1. Field of the Invention
The present invention pertains to an apparatus for connection to the distal end of a tube for insertion in a tubing string or pipe string in a wellbore and for locating the lower end of the tubing string by measuring a change in fluid pressure within the tube to which the apparatus is connected.
2. Background
Many wellbore operations require accurate depth determination, particularly in connection with so-called coiled tubing operations. The accurate determination of the depth of a device connected to the lower end of the coiled tubing string can be particularly critical for operations such as setting inflatable packers and bridge plugs, performing underreaming operations and performing squeeze cementing operations using coilable tubing. Many of these operations are required to be carried out within a few feet of existing wellbore structures such as casing perforations to effectively isolate a zone within an earth formation.
Conventional methods for determining the depth of a tool or device connected to the end of a tubing string while being inserted within another tubing or pipe string within a wellbore have certain shortcomings. For example, devices are known which rely on a change in the reading of the tubing string tension or weight indicator to indicate when a tool or other device connected to the end of the coiled tubing has come in contact with a stop or restriction at a known position in the wellbore tube or pipe string. The lower end of the production tubing string or, more precisely, the end of the production tubing tailpipe is often used as a convenient reference point of known depth. Mechanical tubing end or "tail" locator devices rely on engagement with the lower end of the tubing string, which engagement is sensed by a change in the weight indicator on the coiled tubing to which the device is connected. If resistance to movement of the device upon engagement of the tubing tail or distal end is great enough to be read by the tubing weight indicator at the surface, the end of the coiled tubing can be located in reference to a known position downhole.
However, using an increase in tubing tension as an indicator of location downhole is difficult, particularly, in highly deviated wellbores due to the considerable frictional drag encountered during insertion and movement of the coiled tubing. This change in tension sometimes is, when applied to the stress already on the coiled tubing string, enough to exceed the strength of the tubing string. Additionally, this frictional drag can vary considerably and result in erratic weight indicator readings. If the resistance of movement of the tubing end locator mechanism is set at a relatively low value, so as to prevent the mechanism from being lodged in the tubing or pipe string, the mechanism may not provide for a significant change in tubing tension to be read at the weight indicator.
Still further, certain tubing end configurations may be somewhat funnel-shaped to permit re-entry of wireline conveyed devices and other tools. These tubing end shapes do not present significant resistance to the mechanical tubing locator devices to register a change in tension on the tubing string to which the device is connected. Accordingly, there has been a need for improvement in tubing end and other downhole position reference locating devices which would overcome the abovementioned disadvantages of known types of devices.